\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=stealth, scale=1.3]
  \draw [thick, ->] (.8,0)node[left]{$O$} -- (1.985,0) node[below]{$f_\text{固}$} -- (4.4,0) node[below]{$f$};
  \draw [thick, ->] (.8,0) --(.8,3) node[left]{$A$};
  \node at (-0.25+.8,1.5)[rotate=90]{受迫振动的振幅};
  \draw [thick,gray,dashed] (1.985,0) --++ (0,3);
  \draw [very thick,domain=1:4,samples=200,variable=\x] plot (\x,{1.8/sqrt((4-\x*\x)*(4-\x*\x)+0.1*\x*\x)});
\end{tikzpicture}
\end{document}